Over-current detector

ABSTRACT

An over-current detector monitors the current delivered by a switching regulator and provides a signal for disabling the switching regulator when the current exceeds a threshold value. The threshold value is determined by the value of the output voltage from the switching regulator.

I United States Patent 1 1 3,679,964 Nowell 1451 July 25, 1972 [54]OVER-CURRENT DETECTOR 3,526,823 9/ 1970 Genuit ..321/1 1 X [72]Inventor: John R. Nowell, Phoenix, Ariz.

[73] H u I f i Primary Examiner-A. D. Pellinen lgnee. oneywe n ormat onSystems Inc., mmmey uoyd B Gucmsey at Waltham, Mass.

[22] Filed: Oct. 4, 1971 57] ABSTRACT [21] Appl. No.: 186,122

An over-current detector monitors the current delivered by a switchingregulator and provides a signal for disabling the [52] U.S. Cl ..323/9,321/1 1, 321/14 i hi regulator when the current exceeds a threshold [51]Int. Cl ..H02h 7/12, l-l02m 3/32 value. The threshold value isdetermined by the value of the [58] Field of Search ..321/2, 1 1, 14;323/9, 22 SC put ltage from the switching regulator.

[56] References Cited 10 Claims, 4 Drawing Figures UNITED STATES PATENTS3,470,449 9/1969 Risberg ..32l/l1 mimsnwm w 3.679.964

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IN V EN TOR.

A GENT OVER-CURRENT DETECTOR BACKGROUND OF THE INVENTION This inventionrelates to switching regulators and more particularly to over-currentdetectors which monitor the current delivered by a switching regulatorand provide a signal for disabling the regulator when the currentexceeds a threshold value.

In high speed data processing systems, microcircuits are used to reducethe physical size of the system and to increase the operating speed.These microcircuits are built in modules each of which may replace alarge number of circuits using discrete circuit components. Each ofthese microcircuit modules requires as much power as several circuitsusing discrete components so that the power required in a single cabinetof a data processing system using microcircuit modules is several timesas large as the power required in a single cabinet using discretecomponents when the two cabinets have the same physical size. Inaddition, high speed microcircuits usually use a much smaller value ofD.C. voltage than circuits employing discrete components. For example,in many high speed microcircuits the required DC. voltage may be lessthan 1 volt. This voltage must be well regulated to provide a constantvalue of DC. for the microcircuits, otherwise variations in DC. voltagemay produce error signals in the data processing system.

The power supplied to any system is the product of the voltage andcurrent so that a power distribution system must supply either a largevalue of current at a relatively small value of voltage or a smallervalue of current at a relatively large value of voltage in order toprovide a large amount of power. In prior art power supplies, power froma 220 volt A.C. line is converted into relatively small values of DC.voltage and large amounts of current are distributed by large conductorsor bus bars to various portions of the data processing system. Thevoltage drop in each bus bar is proportional to the amount of current inthe bus bar so that the value of voltage supplied to each portion of thedata processing system varies as current supplied in that portionvaries. This variation in voltage may produce error signals in the dataprocessing system.

Another disadvantage of the prior art power supplies is that theefficiency of the supplies is very low. These power supplies usuallyinclude a constant voltage transformer, a rectifier which converts theAC. voltage to a DC. voltage, and a series regulator which reduces theDC. voltage to a small but constant value. In such a system the voltagedrop in the constant voltage transformer, the rectifier and the seriesregulator is usually about 5-6 volts, while the output voltage necessaryfor the microcircuits may be as low as one volt. This means that thepower transformer must deliver approximately six to seven times thepower which is used by the microcircuit so that the over all efiiciencyof the power supply is less than percent, thereby causing the powersupply to be bulky and expensive. Because of the large size, these powersupplies are usually located in a separate cabinet and require long busbars to distribute the current to other portions of the data processingsystem. These long bus bars cause severe variations in voltage whencurrent in the bus bar varies. Still another disadvantage of the priorart power supplies is that an excessively large value of current drawnfrom the power supply can cause damage to the series regulator and otherparts of the power supply. Also, a short circuit in the series regulatormay cause a large value of voltage to be supplied to the microcircuitmodules. This large value of voltage can cause damage to themicrocircuits.

A power supply system employing switching regulators and switchingregulator control circuits alleviates the disadvantages of the prior artby converting an AC. voltage to a relatively large value of unregulatedDC. voltage at a plurality of locations in the data processing system.This relatively large value of unregulated DC. voltage can be convertedto a relatively small value of DC voltage by the switching regulators atthe various locations in the data processing system. The switchingregulator has an efficiency which is several times the efficiency ofprior art power supplies employing series regulators thereby causing thephysical size of the switching regulator to be relatively. small andallowing the switching regulator to be positioned near the microcircuitmodules.

The switching regulator may employ a transformer, a pair of siliconcontrolled rectifiers and a source of signals to convert an unregulatedDC. voltage, such as volts, to an accurately regulated voltage, such as1 volt. The silicon controlled rectifiers are employed as switchesbetween the source of unregulated D'.C. voltage and the transformer. Thesilicon controlled rectifiers are located on the high voltage side ofthe transformer where the current and power losses in these rectifiersare low, thereby causing the switching regulator to have a high degreeof efficiency. The regulated DC. voltage obtained from a secondarywinding on a transformer is supplied to a pair of voltage-outputterminals. The transformer provides isolation between the regulated DC.voltage and the source of unregulated DC. voltage so that a shortcircuit in a silicon controlled rectifier will not cause damage to themicrocircuit modules which provide the load on the switching regulator.

The silicon controlled rectifier is a semi-conductor device having ananode, a cathode and a gate. The silicon controlled rectifier can beused as an ON-OFF switch which can be turned on in a very fewmicroseconds. Normally, the silicon controlled rectifier cannot conductcurrent between anode and cathode thereof until a pulse of currentlarger than a threshold value flows from=gate to cathode. If a positivevoltage difference exists between the anode and the cathode when a pulseof voltage flows from the gate, the silicon controlled rectifier fires;i.e., is rendered conductive and a current will flow from the anode tothe cathode. The rate at which the current flow from anode to cathodeincreases when the silicon controlled rectifier fires must be limited toprevent damage to the rectifier. Once anode-cathode flow commences, thegate has no further control over such current flow. Current flow fromanode to cathode in a rectifier can be terminated only by reducing theanode to cathode current below a holding" or minimum current value. Amore detailed description of the operation of a silicon controlledrectifier can be found in the Silicon Controlled Rectifier Manual, 4thedition, 1967, published by the General Electric Company, Syracuse NewYork.

A signal source is coupled to the voltage-output terminals of theswitching regulator and develops trigger signals whose frequency isdetermined by the value of voltage at the voltageoutput terminal. Thetrigger signals are coupled to the silicon controlled rectifiers in theswitching regulator and cause these rectifiers to deliver energy throughthe transformer to output filter capacitors which are connected to thevoltage-output terminal. The signal source senses any change in thevalue of regulated output voltage and causes a change in the frequencyof the trigger signals delivered to the switching regulator. This changein frequency of the trigger signals causes a change in the duty cycle ofthe switching regulator. The duty cycle is a duration of time thatenergy is delivered to the output filter capacitors compared to thetotal duration of time between. trigger signals. This change in thefrequency of the trigger signals and in the duty cycle causes a changein the quantity of energy which the switching regulator delivers to theoutput filter capacitors so that the voltage at the output terminalreturns to the original value.

It is often desirable to monitor the current which a switching regulatordelivers to a load and to provide a signal which will disable theregulator when the current delivered exceeds a threshold value. In somesystems, it may be desirable that the threshold value of current bedetermined by the value of the output voltage. For example, when theoutput voltage is relatively high it may be desired that the thresholdvalue of current be relatively high. When a lower value o output voltageis delivered by the switching regulator a lower threshold value ofcurrent may be used. What is needed is an over-current detector whosethreshold value of current is determined by the output voltage of theswitching regulator. It is desirable for the regulator to deliver ahigher value of current to the output filter capacitors when power isinitially applied to the switching regulator. This higher initial valueof current causes the voltage at the output terminal of the regulator toquickly increase to the regulated value. The over-current detector mustbe temporarily disabled to prevent shut-down of the switching regulatorwhile this higher initial value of current is being delivered. It isdesirable for the over-current detector to disable the switchingregulator whenever a short-circuit or fault condition occurs in a loadacross the voltage-output terminals.

It is, therefore, an object of this invention to provide a new andimproved over-current detection circuit for use with a switchingregulator.

Another object of this invention is to provide an over-current detectorwhich provides a signal which disables the switching regulator when thecurrent from the regulator exceeds a threshold value.

' A further object of this invention is to provide an over-currentdetector whose threshold value of current is proportional to the voltagedelivered by a switching regulator.

Another object of this invention is to provide means for disabling' theover-current detector when power is initially applied to the switchingregulator.

Still another object of this invention is to provide an overcurrentdetector having a relatively low threshold value of current when a faultcondition occurs in the switching regulator.

SUMMARY OF THE INVENTION The foregoing objects are achieved in theinstant invention by providing a new and improved over-current detectorfor monitoring the current delivered by a switching regulator withoutdecreasing the value of the voltage delivered to a load. The switchingregulator delivers a predetermined quantity of electricalenergy to anoutput filter for each time a silicon controlled rectifier in theregulator fires. Each time a silicon controlled rectifier fires a pulseof voltage develops across a z secondary winding of the transformer inthe regulator. The invalue of current when a fault causes the outputvoltage to decrease.

Other objects and advantages of this invention will become apparent fromthefollowing description when taken in connection with the accompanyingdrawings.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a schematic drawing of aswitching regulator and its associated control circuits including theinstant invention;

FIG. 2 is a schematic drawing of an embodiment of the instant invention;

FIG. 3 illustrates a magnetization curve which is useful in explainingthe operation of the circuit shown in FIG. 1; and

FIG. 4 illustrates waveforms which are useful in explaining the instantinvention.

DESCRIPTION OF THE PREFERRED EMBODIMENT Referring more particularly tothe drawings by the characters of reference, FIG. 1 discloses a powersupply system which is designed to provide a constant supply of DC.output voltage for a wide range of values of output current and formonitoring the current delivered to a load which may be connected to thesystem. As indicated in FIG. 1, the system comprises a switchingregulator 10, a switching regulator control circuit 11 for providingtrigger signals to switching regulator 10, and a circuit 12 formonitoring the current and the voltage delivered by the power supply.The switching regulator corrtrol'circuit 11 comprises a triggergenerator 14, a rate generator 15, a recovery disable circuit 16 and anerror amplifier 17. The error amplifier 17 detects any change in voltageat the output terminals of the switching regulator and provides a signalwhose value is determined by the change in the output voltage. Thesignal from the error amplifier 17 causes the rate generator 15 todevelop pulses having a frequency which is determined by the value ofthe signal from the amplifier l7. Pulses from the rate generator causethe trigger generator 14 to develop trigger pulses for the switchingregulator. The recovery disable circuit 16 senses the time that outputcurrent is being delivered by the switching regulator to the outputfilter capacitors and prevents a rate generator from delivering pulsesduring the time that the current is being delivered. I

The circuit 12 for monitoring includes a fault shutdown circuit 19, anover-current detector 20, an over-voltage detector 21 and anunder-voltage detector 22. The over-current detector 20, theover-voltage detector 21 and the under-voltage detector 22 sense anyabnormal values of current or voltage at the output of the switchingregulator and provide signals to the fault shutdowncircuit 19. When thefault shutdown circuit 19 receives a signal from any of the detectors20, 21, and 22 it provides a signal to the rate generator which disablesthe rate generator and prevents any pulses from being supplied totrigger the switching regulator.

SWITCHING REGULATOR As indicated in FIG. 1, switching regulator 11includes a pair of transformers 25 and 26, each having a primary windingand a secondary winding. The primary windings 28 and 30 are connected inseries and are coupled to the high voltage unregulated D.C. power supplyhaving a positive output terminal 36 and a negative output terminal 37.Apair of silicon con trolled rectifiers 33 and 34 control the currentsupplied by the power supply to the primary windings of transformers 25and 26. The anode of silicon controlled rectifier 33 is connected to thepositive terminal-36 of the unregulated DC. power supply and the cathodeof silicon controlled rectifier 33 is connected to the upper end ofprimary winding 28. The gate of silicon controlled rectifier 33 isconnected to one lead of the trigger generator 14 which provides triggersignals to render rectifier 33 conductive. The anode of siliconcontrolled rectifier 34 is connected to the lower end of primary winding30 and the cathode of silicon controlled rectifier 34 is connected tothe negative terminal of the unregulated DC. power supply. A

' second lead from the trigger regulator 14 is connected to the gate ofsilicon controlled rectifier 34 to provide trigger signals to renderrectifier 34 conductive.

The magnetic core employed in transformers 25 and 26 produces themagnetization characteristics illustrated in the magnetization curve ofFIG. 3. The magnetizing force H is equal to the product of the number ofturns in a winding on the transformer core and the number of amperes ofcurrent for each turn of wire divided by the length of the core. Sincethe physical length of the particular transformer core is constant themagnetizing force of the transformer is often expressed as the number ofamperes times the number of turns, or ampere-turns." The flux density Bis the number of lines of flux per square centimeter of the transformercore and is determined by the value of the magnetizing force and thetype of material used in the core. A discussion of the magnetizationcurves can be found in the text book Magnetic Circuits and Transformers"by EB. Staff, M.I.T., 1943, published by John Wiiey & Sons, New York,New York.

The operathn of the circuit of FIG. 1 will now be discussed in connection with the magnetization curve shown in-FIG. 3 and the waveformsshown in FIG. 4.

A pair of capacitors 40 and 41 provide predetermined quarrtities ofelectrical energy to the transformers 25 and 26 each time one of thesilicon control rectifiers 33 and 34 is rendered conductive. Each timeone of the silicon controlled rectifiers 33 and 34 is renderednon-conductive the same predetermined quantity of energy is delivered byone of the transformers 25 and 26 through diodes 43 and 44 to a filtercapacitor 48. Prior to the time t, shown irrFlG. 4, capacitor 40 of FIG.1 is charged to the polarity shown in FIG. 1. At time t, a pulse ofcurrent from trigger generator 14 renders silicon control rectifier 33conductive so that the voltage across the capacitor 40 is applied to theprimary winding 28 of transformer 25 causing a current I to flow fromthe upper plate of capacitor 40 through to anode to cathode of rectifier33, through the primary winding 28 to the lower plate of capacitor 40.The current I through primary winding 28 causes a change of flux in thetransformer core and causes the operating point to move from point Atoward point C of the magnetization curve in FIG. 3. This change in fluxproduces a voltage across primary winding 28, which limits the rate ofincrease in current through silicon controlled rectifier 33, thuspreventing possible damage to rectifier 33. A positive voltage appliedto the upper end of primary winding 28 causes the operating point tomove upward from point C toward point D. The distance between point Cand point D is proportional to the product of the voltage applied toprimary winding 28 and the duration of time this voltage is applied.

The voltage applied to the primary winding 28 is magnetically coupledthrough the transformer core to the secondary winding 29. Between timeand time 2 secondary winding 29 has a positive polarity of voltage atthe lower end of the winding and a negative polarity of voltage at theupper end of the winding. At this time, the voltage across the secondarywinding 29 causes diode 43 to be back biased so that no current flowsthrough the diode or through the secondary winding 28. Capacitor 40provides current until this capacitor has discharged at time t, as shownin waveform E of FIG. 4. The area M under the curve of waveform E (FIG.4) between time t, and t is the sum of the products of the voltageapplied to primary winding 28 and the duration of the time the voltageis applied and this area M represents the total energy stored in thecore of transformer 25. When the voltage applied to primary winding 28has a zero value at time the operating point on the magnetization curveof FIG. 3 reaches point D.

At time t,, the energy stores in the core of transformer 25 reverses thepolarity of voltage across each of the transformer windings so thatnegative polarity of voltage is developed at the upper end of primarywinding 28. This negative polarity of voltage at the upper end ofprimary winding 28 causes the operating point in FIG. 3 to move frompoint D to point E and to begin moving toward point A. Again thedistance between point E and point A is proportional to the product ofthe voltage across primary winding 28 and the duration of time thisvoltage is applied. The area N under the curve of waveform E betweentimes t and I is the sum of the products of voltage across primarywinding 28 and the time this voltage is applied. In this area Nrepresents a total energy which the core of transformer 28 returnsthrough the transformer. The voltage across primary winding 28 causescurrent I to charge capacitor 40 to a polarity opposite to the polarityshown in FIG. 1. The energy in the core of transformer 25 causes thevoltage across secondary winding 29 to increase to a value larger thanthe voltage across filter capacitor 48 so that a current I flows throughdiode 43 to charge capacitor 48. The energy which is stored in the coreof the transformer 25 when silicon controlled rectifier 33 conducts isproportional to the difference between the flux at point A and point Don the magnetization curve of FIG. 3; and the energy which istransferred to the secondary winding 29 when silicon controlledrectifier 33 is rendered non-conductive, is proportional to thedifference between the flux at point E and point A.

Since the distance between point A through point C to point D shown inFIG. 3 is substantially the same as the distance between points Bthrough point F to point A, substantially all of the energy which wasstored in the core of the transformer between times and t, is returnedand is stored in capacitors 48 and 49. Capacitor 40 deliverssubstantially the same amount of energy to the transformer each time thesilicon controlled rectifier 33 is rendered conductive so that theamount of energy delivered to filter capacitors 48 and 49 and thevoltage across these capacitors is determined by the frequency of thesignals applied to the gate of rectifier 33.

Capacitor 41 also provides a predetermined quantity of energy to thetransformer 26 each time silicon controlled rectifier 34 is renderedconductive. Prior to time I capacitor 41 is charged to the polarityshown in FIG. 1. At time t, a pulse of current from the triggergenerator 14 renders silicon controlled rectifier 34 conductive so thatcurrent I, flows from the upper plate of capacitor 41 through theprimary winding 30, from anode to cathode of rectifier 34 to the lowerplate of capacitor 41. Current I, through the primary winding and thevoltages impressed across this winding cause the operating point of thecharacteristic curve in FIG. 3 to move from point A through point C topoint D and causing a predetermined quantity of energy to be stored inthe core of transformer 26. When silicon controlled rectifier 34 isrendered non-conductive, this energy is transferred through thesecondary winding 31 causing a current I, to charge capacitor 48 asdescribed above.

The amount of voltage across the capacitors 48 and 49 can be controlledby controlling the frequency of the trigger signals which triggergenerator 14 applies to the gates of silicon controlled rectifiers 33and 34. The frequency of the trigger signals is determined by the valueof the current applied to the rate generator 15. When an increase in theamount of current drawn by a load (not shown) connected across theoutput terminals 51 and 52 in FIG. 1 causes the value of the outputvoltage to fall below a predetermined reference level, the frequency ofthe signals from trigger generator 14 increases. This increase in thefrequency of the output signals causes an increase in the rate of energydelivered to filter capacitors 48 and 49 and increases the voltage atthe output terminals 51 and 52 to the predetermined reference level. Thevoltage at the output terminal 51 of the power supply controls thefrequency of the signals from the trigger generator 14 so that thevoltage at the output terminals 51 and 52 is substantially constant evenwhen the current drawn from this power supply varies over a wide rangeof values. A more detailed description of the operation of the switchingregulator can be found in the US. Pat. No. 3,518,526 by Luther L.Genuit, issued June 30, 1970, entitled Switching Regulator.

CURRENT-DETECTOR CIRCUIT As indicated in FIG. 2, the circuit fordetecting the value of current delivered by the switching regulatorcomprises a reference amplifier 55, a current monitor circuit 56, anamplifier 57 and a fault shut down circuit 58. The reference amplifier55 includes a transistor 60 having a base, a collector and an emitter. Afirst reference voltage is applied to an input terminal 61 and appliedto a voltage divider network comprising resistor 62 and 63. This voltagedivider network provides a voltage at output terminal 71 which is lessthan the voltage at input terminal 61. In applications where it isdesired that the maximum current from the switching regulator (FIG. 1)be determined by the output voltage from terminals 51 and 52 of FIG. 1,terminals 61 and 66 of the over-current detector of FIG. 2 are connectedto terminals 51 and 52 respectively of the switching regulator shown inFIG. 1. When it is desired that the over-current detector have aconstant value of maximum or threshold current, terminal 61 of theover-current detector is connected to a constant source of referencepotential. Transistor 60 provides isolation between the junction point67 and the emitter of the transistor 68 so that the reference voltage atjunction point 67 does not change when the transistor 68 is renderedconductive.

A capacitor 69 and a rheostat 70 are connected in parallel between thebase of transistor 68 and a reference potential, such as ground. A diode72 is connected between the base of transistor 68 and resistor 74 whichis connected to a terminal 77. Terminal 77 is connected to the positiveoutput terminal 36 (FIG. 1) of the high voltage unregulated DC. powersupply so that the voltage applied to resistor 74 is the same as thevoltage (+V,,,) applied to the input terminals of the switchingregulator. In some circuits the value of the voltage +V.,, may be largerthan the voltage desired at terminal 77. In these circuits a voltagedivider network may be used to reduce the value of the voltage appliedto terminal 77. Another diode 79 is connected between the anode of diode72 and the secondary 31 of transformer 26 (FIG. I). The voltage appliedto ter-, minals 80 and 81 is the voltage which is developed across thesecondaryof transformer 26 in the switching regulator of FIG. 1.

The operation of the over-current detector circuit of FIG. 2 will now bediscussed in connection with the switching regulator circuit shown inFIG. 1 and the waveforms shown in FIG. 4.

The current delivered to the output capacitors 48 and 49 (FIG. 1) bytransformers 25 and 26 respectively are shown in waveforms I and I, ofFIG. 4. The average value of the current delivered by each of thesetransformers is approximately equal to b (1, X (duty cycle). The valueof I,, is determined by the value of the voltage, +V at terminal 36. Ifthe duty cycle is increased so that the pulses of current such as thoseshown at times t, and t of waveform I are closer together the averagevalue of the current delivered to the output capacitors isincreased.Thus, the duty cycle of the switching regulator and the value of thevoltage +V,,, can be used to measure the value of current which isdelivered by the switching regulator to the output filter capacitors.The duty cycle can be determined by using the voltage E, as a signalbetween terminals 80 and 81 of FIG. 2. Voltage +V,,, at terminal 77is-used to sense the value of current I Prior to the time t, (FIG. 4)the voltage across secondary winding 3] at terminal 80 (FIG. 2) has avalue of zero so that thevoltage at the junction point 75 has a lowvalue of voltage. When junction point 75 has a low value of voltagecapacitor 69 discharges through rheostat 70. At time t, the voltage atterminal 80 and the voltage at junction point 75 has a positive value sothat a current I., flows from terminal 77 through resistor 74 and diode72 to the upper plate of capacitor 69, thereby providing a charge oncapacitor 69. The value of current I, is determined by the value ofvoltage +V,,, at terminal 77 and by the value of resistor 74. Due to theshort duration of time between t-, and t capacitor 69 does not charge tothe full value of voltage which is present at terminal 80. Between timest and I a current I, flows from the upper plate of capacitor 69 throughrheostat 70 to the lower plate of capacitor 69, thereby partiallydischarging capacitor 69. Wheir'the duty cycle is small the timeduration between time t and time 1 is relatively long so that capacitor69 discharges for a relatively long time thereby removing most of thecharge from capacitor 69. However, whenthe duty cycle increases, thetime between and time is reduced so that capacitor 69 has a larger valueof charge at the end of the discharge period of time. At time rcapacitor 69 again charges through resistor 74 and diode 72 to a greatervalue than it charged the previous time, due to the remaining charge oncapacitor 69 at the start of the charging period. When the voltageacross capacitor 69 reaches a value which is greater than the voltageacross re- .sistor 65, transistor 68 is rendered conductive. Whentransistor 68 is rendered conductive a current I flows from terminal 88through resistor 85, through transistor 68 and resistor 65 to ground.

Current 1,, through resistor 85 provides the potential shown acrossresistor 85 so that transistor 84 is rendered conductive. Whentransistor 84 is rendered conductive a current I, flows fromterminal 88through emitter to collector of transistor 85, through resistor 86,through gate to cathode junction of silicon controlled rectifier 90 toground, thereby rendering silicon controlled rectifier 90 conductive.When silicon controlled rectifier 90 is rendered conductive a current Iflows from terminal 95 through resistor 94 and silicon controlledrectifier 90 to ground. Current I provides a voltage drop acrossresistor 94 so that the voltage at the anode of the silicon controlledrectifier decreases. The decrease in voltage at the anode of rectifier90 provides a low value of signal through diode 97 to the signal-outputterminal 98. The signal at output terminal 98 is coupled to the rategenerator (FIG. 1) and causes the switching regulator to be disabled.

Capacitor 69 prevents the over-current detector from disabling theswitching regulator when power is initially applied to theregulator'sothat the regulator can supply a relatively large current tocharge the output filter capacitors. When power is initially applied tothe switching regulator capacitor 69 is discharged. Capacitor 69 has arelatively large value, such as 22 microfards, so that several chargeperiods are required to increase the voltage across capacitor 69 from avalue of zero to a value which will render transistor 68 con ductive.During the time that capacitor 69 is charging transistor 68 isnonconductive so that transistor 84 and silicon controlled rectifier arenon conductive and the switching regulator will'not be disabled by asignalfrom terminal 98. Thus, when power is initially applied to theregulator the duty cycle can be high and current to the output filtercapacitors 48 and 49 (FIG. I) can be high without causing the switchingregulator to be disabled.

When a short occurs in the load at the output terminals of the switchingregulator the output voltage, +V,, decreases. This causes a decrease inthe reference voltage at the base of transistor 60 and at the emittersof transistors 60 and 68 (FIG.

2). A relatively low value of voltage across capacitor 69 causestransistor 68 to be rendered conductive and to provide a signal whichdisables the switching regulator. Thus, the overcurrent detectorprotects the regulator from damage caused by short circuits in the load.

The over-current detector of FIG. 2 has a calibration circuit whichincludes resistor 73 and switch 76. In a typical circuit the value ofresistor 73 may be 10 times the value of resistor 74. Switch 76 isclosed while the circuit is being calibrated and the switch is openduring normal operation of theswitching regulator. When switch 76 isclosed the current through resistor 73 is equal to 10 percent of thevalue of current flowing through resistor 74 when capacitor 69 ischarging. This causes capacitor 69 to charge at a rate of 10 percentfaster than when switch 76 is open. To calibrate the over-currentdetector a load which draws a desired normal value of current is placedacross terminals 51 and 52 of the switching regulator of FIG. 1 andswitch 76 is closed. The value of rheostat 70 is slowly increased from alow value until a disable signal is produced at terminal 98 of FIG. 2.When switch 76 is open an output current of 10 percent above normalvalue must be drawn from the switching regulator and a duty cycle of 10percent above normal produced to provide the extra time for capacitor 69to charge enough to render transistor 68 conductive and disable theswitching regulator. Other ratios of values of resistors 73 and 74 canbe used to obtain different threshold values of output currents from theswitching regulator.

While the principles of the invention have now been made clear in anillustrative embodiment, there will be immediately obvious to thoseskilled in the art many modifications of structure, arrangement,proportions, the elements, materials, and components, used in thepractice of the invention, and otherwise, which are particularly adaptedfor specific environments and operating requirements without departingfrom those principles. The appended claims are therefore intended tocover and embrace any such modifications, within the limits only of thetrue spirit and scope of the invention.

I claim:

1. An over-current detector for use with a switching regulator whichincludes a transformer having a primary winding and a secondary winding,said detector comprising:

a first transistor having a base, a collector and an emitter;

first, second, third and fourth reference potentials;

first, second and third resistors, said first resistor being connectedbetween said first potential and said collector of said firsttransistor, said second resistor being connected between said secondpotential and said base of said first transistor;

first and second diodes each having an anode and a cathode, said cathodeof said first diode being connected to said base of said firsttransistor, said third resistor being connected between said thirdpotential and said anode of said first diode, said anode of said seconddiode being connected to said anode of said first diode, said secondaryof said transformer being connected between said second potential andsaid cathode of said second diode;

a capacitor, said capacitor being connected between said secondpotential and said base of said first transistor;

said fourth potential being coupled to said emitter of said firsttransistor; and

an output lead, said output lead being coupled to said collector of saidfirst transistor;

2. An over-current detector as defined in claim 1, including:

a second transistor having a base, a collector and an emitter;

and

a fourth resistor, said fourth resistor being connected between saidsecond potential and said emitters of said first and said secondtransistors, said base of said second transistor being coupled to saidfourth potential, said collector of said second transistor beingconnected to said first potential.

3. An over-current detector as defined in claim 2, including:

a third transistor having a base, a collector and an emitter, said baseof said third transistor being connected to said output lead, saidemitter of said third transistor being connected to said firstpotential;

a controlled rectifier having an anode, a cathode and a gate, said gateof said rectifier being coupled to said collector of said thirdtransistor, said cathode of said rectifier being connected to saidsecond potential;

a fifth resistor, said fifth resistor being connected between said firstpotential and said anode of said rectifier; and

a signal-output terminal, said signal-output terminal being coupled tosaid anode of said rectifier.

4. An over-current detector for use with a switching regulator whichincludes first and second voltage-output terminals and a transformerhaving a secondary winding, said detector comprising:

first and second transistors each having a base, a collector and anemitter;

first, second and third reference potentials;

first, second, third and fourth resistors, said first resistor beingconnected between said first potential and said collector of said firsttransistor, said second resistor being connected between said secondpotential and said base of said first transistor;

a voltage divider network having first and second input terminals and anoutput terminal, said first input terminal of said network beingconnected to said first output terminal of said regulator, said secondinput terminal of said network being connected to said second outputterminal of said regulator, said output terminal of said network beingconnected to said base of said second transistor, said second outputterminal of said regulator being connected to said second potential;

first and second diodes each having an anode and a cathode, said cathodeof said first diode being connected to said base of said firsttransistor, said third resistor being connected between said thirdpotential and said anode of said first diode, said anode of said seconddiode being connected to said anode of said first diode, said secondaryof said transformer being connected between said second potential andsaid cathode of said second diode;

a capacitor, said capacitor being connected between said secondpotential and said base of said first transistor, said fourth resistorbeing connected between said second potential and said emitters of saidfirst and said second transistors, said collector of said secondtransistor being connected to said first potential; and

an output lead, said output lead being coupled to said collector of saidfirst transistor.

5. An over-current detector as defined in claim 4 including:

a third transistor having a base, a collector and an emitter, said baseof said third transistor being connected to said output lead, saidemitter of said third transistor being connected to said firstpotential;

a controlled rectifier having an anode, a cathode and a gate, said gateof said rectifier being coupled to said collector of said thirdtransistor, said cathode of said rectifier being connected to saidsecond potential;

a fifth resistor, said fifth resistor being connected between said firstpotential and said anode of said rectifier; and

a signal output-terminal, said signal-output terminal being coupled tosaid anode of said rectifier.

6. An over-current detector as defined in claim 4, including:

a switch having first and second terminals;

fifth and sixth resistors, said first terminal of said switch beingconnected to said third potential, said sixth resistor being connectedbetween said second terminal of said switch and said anode of said firstdiode;

a third transistor having a base, a collector and an emitter, said baseof said third transistor being connected to said output lead, saidemitter of said third transistor being connected to said firstpotential;

a controlled rectifier having an anode, a cathode and a gate, said gateof said rectifier being coupled to said collector of said thirdtransistor, said cathode of said rectifier being connected to saidsecond potential, said fifth resistor being connected between said fustpotential and said anode of said rectifier; and

a signal-output terminal, said signal-output terminal being coupled tosaid anode of said rectifier.

7. An over-current detector for use with a switching regulator whichincludes first and second voltage-input terminals,

first and second voltage-output terminals and a transformer having asecondary winding, said detector comprising:

first and second transistors each having a base, a collector and anemitter;

first and second reference potentials, said collector of said secondtransistor being connected to said first potential;

a voltage divider network having first and second input terminals and anoutput terminal, said first input terminal of said network beingconnected to said first output terminal of said regulator, said secondinput terminal of said network being connected to said second outputterminal of said regulator and to said second input terminal of saidregulator, said output terminal of said network being connected to saidbase of said second transistor, said second output terminal of saidregulator being connected to said second potential;

first, second, third and fourth resistors, said first resistor beingconnected between said first potential and the said collector of saidfirst transistor, said second resistor being connected between saidsecond potential and said base of said first transistor;

first and second diodes each having an anode and a cathode, said cathodeof said first diode being connected to said base of said firsttransistor, said third resistor being connected between said anode ofsaid first diode and said first input terminal of said switchingregulator, said anode of said second diode being connected to said anodeof said first diode, said secondary winding of said transfonner beingconnected between said second potential and said cathode of said seconddiode;

a capacitor, said capacitor being connected between said secondpotential and said base of said first transistor, said fourth resistorbeing connected between said second potential and said emitters of saidfirst and said second transistors; and

an output lead, said output lead being coupled to said collector of saidfirst transistor.

8. An over-current detector as defined in claim 7 including:

a third transistor having a base, a collector and an emitter, said baseof said third transistor being connected to said output lead, .saidemitter of said third transistor being connected to said firstpotential;

a controlled rectifier having an anode, a cathode and a gate, said gateof said rectifier being coupled to said collector of said thirdtransistor, said cathode of said rectifier being connected to saidsecond potential;

a fifth resistor, said fifth resistor being connected between said firstpotential and said anode of said rectifier; and

a signal-output terminal, said signal-output terminal being coupled tosaid anode of said rectifier. I

9. An over-current detector as defined in claim 7 including:

a switch having first and second terminals; and

a sixth resistor, said first terminal of said switch being connected tosaid first input terminal of said regulator, said sixth resistor beingconnected between said second terminal of said switch and said anode ofsaid first diode.

10. An over-current detector as defined in claim 7 includa switch havingfirst and second terminals;

fifth and sixth resistors, said first terminal of said switch beingconnected to said first input terminal of said regulator, said sixthresistor being connected between said second terminal of said switch andsaid anode of said first diode;

a third transistor having a base, a collector and an emitter, said baseof said third transistor being connected to said output lead, saidemitter of said third transistor being connected to said firstpotential;

a controlled rectifier having an anode, a cathode and a gate, said gateof said rectifier being coupled to said collector of said thirdtransistor, said cathode of said rectifier being connected to saidsecond potential, said fifth resistor being connected between said firstpotential and said anode of said rectifier; and

a signal-output terminal, said signal-output terminal being coupled tosaid anode of said rectifier;

e a r a t

1. An over-current detector for use with a switching regulator whichincludes a transformer having a primary winding and a secondary winding,said detector comprising: a first transistor having a base, a collectorand an emitter; first, second, third and fourth reference potentials;first, second and third resistors, said first resistor being connectedbetween said first potential and said collector of said firsttransistor, said second resistor being connected between said secondpotential and said base of said first transistor; first and seconddiodes each havinG an anode and a cathode, said cathode of said firstdiode being connected to said base of said first transistor, said thirdresistor being connected between said third potential and said anode ofsaid first diode, said anode of said second diode being connected tosaid anode of said first diode, said secondary of said transformer beingconnected between said second potential and said cathode of said seconddiode; a capacitor, said capacitor being connected between said secondpotential and said base of said first transistor; said fourth potentialbeing coupled to said emitter of said first transistor; and an outputlead, said output lead being coupled to said collector of said firsttransistor;
 2. An over-current detector as defined in claim 1,including: a second transistor having a base, a collector and anemitter; and a fourth resistor, said fourth resistor being connectedbetween said second potential and said emitters of said first and saidsecond transistors, said base of said second transistor being coupled tosaid fourth potential, said collector of said second transistor beingconnected to said first potential.
 3. An over-current detector asdefined in claim 2, including: a third transistor having a base, acollector and an emitter, said base of said third transistor beingconnected to said output lead, said emitter of said third transistorbeing connected to said first potential; a controlled rectifier havingan anode, a cathode and a gate, said gate of said rectifier beingcoupled to said collector of said third transistor, said cathode of saidrectifier being connected to said second potential; a fifth resistor,said fifth resistor being connected between said first potential andsaid anode of said rectifier; and a signal-output terminal, saidsignal-output terminal being coupled to said anode of said rectifier. 4.An over-current detector for use with a switching regulator whichincludes first and second voltage-output terminals and a transformerhaving a secondary winding, said detector comprising: first and secondtransistors each having a base, a collector and an emitter; first,second and third reference potentials; first, second, third and fourthresistors, said first resistor being connected between said firstpotential and said collector of said first transistor, said secondresistor being connected between said second potential and said base ofsaid first transistor; a voltage divider network having first and secondinput terminals and an output terminal, said first input terminal ofsaid network being connected to said first output terminal of saidregulator, said second input terminal of said network being connected tosaid second output terminal of said regulator, said output terminal ofsaid network being connected to said base of said second transistor,said second output terminal of said regulator being connected to saidsecond potential; first and second diodes each having an anode and acathode, said cathode of said first diode being connected to said baseof said first transistor, said third resistor being connected betweensaid third potential and said anode of said first diode, said anode ofsaid second diode being connected to said anode of said first diode,said secondary of said transformer being connected between said secondpotential and said cathode of said second diode; a capacitor, saidcapacitor being connected between said second potential and said base ofsaid first transistor, said fourth resistor being connected between saidsecond potential and said emitters of said first and said secondtransistors, said collector of said second transistor being connected tosaid first potential; and an output lead, said output lead being coupledto said collector of said first transistor.
 5. An over-current detectoras defined in claim 4 including: a third transistor having a base, acollector and an emitter, said base of said third transistor beinGconnected to said output lead, said emitter of said third transistorbeing connected to said first potential; a controlled rectifier havingan anode, a cathode and a gate, said gate of said rectifier beingcoupled to said collector of said third transistor, said cathode of saidrectifier being connected to said second potential; a fifth resistor,said fifth resistor being connected between said first potential andsaid anode of said rectifier; and a signal output-terminal, saidsignal-output terminal being coupled to said anode of said rectifier. 6.An over-current detector as defined in claim 4, including: a switchhaving first and second terminals; fifth and sixth resistors, said firstterminal of said switch being connected to said third potential, saidsixth resistor being connected between said second terminal of saidswitch and said anode of said first diode; a third transistor having abase, a collector and an emitter, said base of said third transistorbeing connected to said output lead, said emitter of said thirdtransistor being connected to said first potential; a controlledrectifier having an anode, a cathode and a gate, said gate of saidrectifier being coupled to said collector of said third transistor, saidcathode of said rectifier being connected to said second potential, saidfifth resistor being connected between said first potential and saidanode of said rectifier; and a signal-output terminal, saidsignal-output terminal being coupled to said anode of said rectifier. 7.An over-current detector for use with a switching regulator whichincludes first and second voltage-input terminals, first and secondvoltage-output terminals and a transformer having a secondary winding,said detector comprising: first and second transistors each having abase, a collector and an emitter; first and second reference potentials,said collector of said second transistor being connected to said firstpotential; a voltage divider network having first and second inputterminals and an output terminal, said first input terminal of saidnetwork being connected to said first output terminal of said regulator,said second input terminal of said network being connected to saidsecond output terminal of said regulator and to said second inputterminal of said regulator, said output terminal of said network beingconnected to said base of said second transistor, said second outputterminal of said regulator being connected to said second potential;first, second, third and fourth resistors, said first resistor beingconnected between said first potential and the said collector of saidfirst transistor, said second resistor being connected between saidsecond potential and said base of said first transistor; first andsecond diodes each having an anode and a cathode, said cathode of saidfirst diode being connected to said base of said first transistor, saidthird resistor being connected between said anode of said first diodeand said first input terminal of said switching regulator, said anode ofsaid second diode being connected to said anode of said first diode,said secondary winding of said transformer being connected between saidsecond potential and said cathode of said second diode; a capacitor,said capacitor being connected between said second potential and saidbase of said first transistor, said fourth resistor being connectedbetween said second potential and said emitters of said first and saidsecond transistors; and an output lead, said output lead being coupledto said collector of said first transistor.
 8. An over-current detectoras defined in claim 7 including: a third transistor having a base, acollector and an emitter, said base of said third transistor beingconnected to said output lead, said emitter of said third transistorbeing connected to said first potential; a controlled rectifier havingan anode, a cathode and a gate, said gate of said rectifier beingcoUpled to said collector of said third transistor, said cathode of saidrectifier being connected to said second potential; a fifth resistor,said fifth resistor being connected between said first potential andsaid anode of said rectifier; and a signal-output terminal, saidsignal-output terminal being coupled to said anode of said rectifier. 9.An over-current detector as defined in claim 7 including: a switchhaving first and second terminals; and a sixth resistor, said firstterminal of said switch being connected to said first input terminal ofsaid regulator, said sixth resistor being connected between said secondterminal of said switch and said anode of said first diode.
 10. Anover-current detector as defined in claim 7 including: a switch havingfirst and second terminals; fifth and sixth resistors, said firstterminal of said switch being connected to said first input terminal ofsaid regulator, said sixth resistor being connected between said secondterminal of said switch and said anode of said first diode; a thirdtransistor having a base, a collector and an emitter, said base of saidthird transistor being connected to said output lead, said emitter ofsaid third transistor being connected to said first potential; acontrolled rectifier having an anode, a cathode and a gate, said gate ofsaid rectifier being coupled to said collector of said third transistor,said cathode of said rectifier being connected to said second potential,said fifth resistor being connected between said first potential andsaid anode of said rectifier; and a signal-output terminal, saidsignal-output terminal being coupled to said anode of said rectifier.